A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
There is a desire to integrate an ever-increasing number of electronic components in an IC. To realize this it is necessary to decrease the size of the components and therefore to increase the resolution of the projection system, so that increasingly smaller details, or line widths, can be projected on a target portion of the substrate. For the projection system this means that the projection system and the lens elements used in the projection system must comply with very stringent quality requirements. Despite the great care taken during the manufacturing of lens elements and the projection system they both may still suffer from wave front aberrations, such as, for example, displacement, defocus, astigmatism, coma and spherical aberration across an image field projected with the projection system onto a target portion of the substrate. Said aberrations are important sources of variations of the imaged line widths occurring across the image field. It is important that the imaged line widths at different points within the image field are constant. If the line width variation is large, the substrate on which the image field is projected may be rejected during a quality inspection of the substrate. Using techniques such as phase-shifting masks, or off-axis illumination, the influence of wave front aberrations on the imaged line widths may further increase.
After the projection system has been built into a lithographic projection apparatus, the wave front aberrations may need to be measured. Moreover, since wave front aberrations may vary in time in a projection system, for instance, due to deterioration of the lens material or lens heating effects (local heating of the lens material), it may be necessary to measure the aberrations at certain instants in time during operation of the apparatus and to adjust certain movable lens elements accordingly to minimize wave front aberrations. The short time scale, on which lens-heating effects may occur, may require measuring the wave front aberrations frequently. The measured wave front aberrations may be expressed, for example, in terms of Zernike coefficients.
In a lithographic apparatus it can be important to know other characteristics of the projection system used for imaging the pattern on the patterning device onto the substrate, as well as or instead of the wave front aberrations. Such characteristics may also be referred to as properties or parameters of the projection system. One such property is the numerical aperture (NA) of the lens system, which affects the imaging of the lithographic apparatus. Knowledge of the exact value of the numerical aperture can be used in simulations to determine settings and process windows for the lithographic apparatus. In some apparatus, the projection system has an adjustable numerical aperture that is defined by means such as an adjustable diaphragm at a pupil plane in the projection lens system. Measurement of the actual numerical aperture setting is thus important.
Another characteristic that it can be desirable to evaluate is the telecentricity of the projection lens system. Non-telecentricity of the projection system affects the imaging performance and can cause overlay problems.
The demand for ever-smaller features to be imaged with lithographic apparatus has further resulted in the use of projection systems with increasing numerical aperture (NA). The angle of rays of radiation within the projection apparatus with respect to the optical axis increases with increasing NA. The vector nature of light becomes important for imaging because only identically polarized components of electromagnetic waves interfere. Therefore it is not the wave front quality alone that determines the image contrast; the polarization has a considerable influence as well. Furthermore, the use of illumination radiation having specifically desired states of polarization for specific regions is increasingly being used for imaging features aligned in particular directions. Consequently, it is desirable to know the state of polarization of the radiation impinging on the patterning device, such as a reticle, and it is desirable to know, as another property of the projection system, the effect on the state of polarization caused by the projection system, for example expressed as Jones matrices.
Various apparatus has been described for measuring the above properties. Typically, such apparatus consists of a source module positionable in the beam path at the level of the reticle for conditioning the radiation, for example by providing a desired spatial intensity distribution, angular intensity distribution and/or polarization. The radiation then traverses the projection system and is then incident on a sensor unit at the wafer level or in the wafer table of the lithographic apparatus. The sensor unit contains a detector, either a single detector or an array of multiple detectors, for radiation intensity measurements. The source module and sensor unit usually comprise further optical elements, such as pinholes, gratings, lenses, birefringent waveplates, and so forth, as necessary for measuring the desired property of the projection system.
However, there is a problem because of lack of space in the lithographic apparatus, particularly at wafer level, for installing the sensor unit. This problem is even more acute when trying to retro-fit a sensor unit in an older apparatus that is only capable of handling eight inch (200 mm) wafers or smaller.
A further problem is that of heat dissipation by the sensor unit in the wafer table. The detector electronics inevitably generate heat, and heat dissipation is very critical with respect to overlay and focus performance of the apparatus.
A yet further problem is the constraint on connectivity with the sensor unit at wafer level; i.e. the problem of physically routing wires to the sensor unit and connecting them to other electronics in the apparatus, for example for supplying power and control signals to the source module, and for communication of the detection results to a computer and/or storage device for obtaining the measurement of the desired properties.